1. Field of the Invention
This invention relates to improvements to direct current electromagnetic contactors equipped with magnetic arc-extinguishing means.
2. Description of the Prior Art
Heretofore, direct current electromagnetic contactors with magnetic arc-extinguishing means have typically comprised a stationary coil resiliently mounted in an insulating base and surrounded by a coil to which electric power may be supplied to energize the coil and generate an electromagnetic force in the stationary core, which thereby attracts a movable core which is mechanically linked to a cross-bar means supporting normally closed and/or normally open contact sets which switch DC currents flowing in a main circuit. The normally closed or normally open contact sets are respectively switched to the opposite state by the movement of the cross-bar due to the electromagnetic attraction between the stationary core and the movable core when the coil surrounding the stationary core is energized. The same contacts are returned to their normal states when the coil is deenergized to cease the electromagnetic force and release the movable core and the cross-bar to be returned to their original positions by the force of a return spring.
On the occasion of the opening of any of the contact sets, either by opening the normally closed contacts, or opening the normally open contacts after they have been closed by movement of the cross-bar, it is usual that an arc will be produced between the opening contacts. This arc, if not controlled, can damage the contacts and may interfere with or prevent the interruption being effected. Therefore the prior art typically included magnetic arc-extinguishing devices each consisting of a permanent magnet surrounded by a protective sleeve for protecting the permanent magnet against arc damage, with pole plates sandwiching and supporting the permanent magnet at right angles to the direction of magnetization, and a connecting spar linking the two pole plates.
By virtue of provision of these magnetic arc-extinguishing devices, the arcs that are produced at right angles to the direction of the magnetic field formed by the permanent magnets, in accordance with Fleming's left-hand rule, and are thus drawn out and extinguished.
However, in a prior device of the construction described above, the number of parts making up the magnetic arc-extinguishing devices is great, and as a magnetic arc extinguishing device must be disposed for each set of normally open or normally closed contacts, the drawback of a high basic cost arises. Further, the various contacts of the normally open and the normally closed contact sets are closely adjacent to the pole plates of the magnetic arc-extinguishing devices and so a metal vapor of the contact material, etc., produced during arcing, causes a reduction in the dielectric strength in the space around the contacts such that the arc may transfer to the pole plates, and in these circumstances, the arc cease to be perpendicular to the flux of the permanent magnet, as a result of which the arc driving force is reduced whereby it may become impossible to effect interruption and the heat produced by the arc may burn the surrounding insulator, such as the frame, with the fear of momentary short circuits. Furthermore, with an increase in current or raised voltage, these problems are magnified, and become more frequent, leading to reduced interruption performance and shortened switching life.